JPH0753741B2 - Room temperature vulcanizable silicone rubber composition - Google Patents

Description

TECHNICAL FIELD This invention relates to self-adhesive-component naturally vulcanizing silicone sealants. Even more specifically, this invention relates to the use of mixed oxime-alkoxysilylalkylurea compounds having two substituents as a fixing agent in sealants.

Background Traditionally, the low modulus properties of silicone rubber have been desirable in accommodating joint movement due to vibration and thermal cycling, because stress is very low on the adhesive interface between sealant and metal. Because. Such low modulus properties generally sense the probability of oil leakage. However, oil resistance has traditionally been improved by utilizing a relatively high crosslink density. As a result, the elongation properties were even lower, and thus the ability to move was diminished. If less cross-linking agent is incorporated into the elastomer to lower the modulus, generally faster degradation of the polymer backbone will occur at higher temperatures due to the nucleophile or electrophile. occured.
Therefore, natural vulcanized elastomers with good oil resistance are
In general, it was never made.

U.S. Pat. No. 3,189,576 relates to oxime curing agents and new organosilicone intermediates.

U.S. Pat. No. 4,323,489 and U.S. Pat. No. 3,962,160 generally relate to oxime-curing natural vulcanizates.
Even more specifically, U.S. Pat.No. 4,323,489 relates to vulcanizable silicone rubber compositions which have a very low modulus, silanol-terminated diorganopolysiloxanes, difunctionalized as chain extenders. Of acetamidocupler and a very small amount of a compound having 1 oxime functionality as a trifunctional crosslinker thereon.

U.S. Pat. No. 4,356,116 relates to low volatility, naturally vulcanized silicone rubber compositions containing various components including silanol polymers, crosslinkers and fillers.

U.S. Pat. No. 3,517,001 relates to a fixer composition.

U.S. Pat. No. 4,100,129 relates to fixer compositions that are curable in the presence of moisture.

U.S. Pat.No. 4,395,526 relates to stable, substantially acid-free, one-component curable polyalkoxy-terminated organopolysiloxane compositions having a condensation catalyst such as a tin compound. It is a thing. The material of this US patent also serves as a fixing agent.

Specific patents relating to acetoxy-curable, naturally vulcanized silicones are U.S. Patents 3,133,891, 3,035,016 and 3,
It is No. 382,205. Even more specifically, U.S. Pat. No. 3,3
No. 82,205 consists of a mixture of an organotriacyloxysilane and a base mixture containing an organosiloxane having combined organosiloxy building blocks.

U.S. Pat. No. 3,541,044 relates to a substantially anhydrous organopolysiloxane composition capable of curing to a rubbery elastic state upon exposure to moisture. U.S. Pat. No. 3,837,878 relates to two-component naturally vulcanized silicone rubber compositions suitable for molding, while U.S. Pat. No. 3,837,878 relates to methods for treating silicone fillers.

US Pat. No. 3,776,933 relates to chain extenders for natural vulcanization systems.

While most of the above patents relate to organopolysiloxane compositions, those patents
There is no teaching of the two component substituted oxime-alkoxysilylalkylurea derivatives as one-component spontaneously vulcanized silicone sealant compositions or fixers of this invention.

DISCLOSURE OF THE INVENTION Accordingly, an aspect of this invention is to provide a fixative.

Furthermore, an aspect of the present invention is to provide the above-mentioned fixing agent, which can be produced via a reaction product of an alkoxysilylalkylamine.

Another aspect of the present invention is to provide a self-adhesive one-component naturally vulcanizing silicone sealant containing an adhesion promoter, the sealant being non-corrosive with respect to iron-containing and aluminum supports. It is a thing.

Yet another aspect of the present invention is to provide a self-adhesive one-component naturally vulcanizing silicone sealant containing the above fixing agent, wherein the sealant is first applied to the support. It does not require a primer.

Still yet another aspect of the present invention is to provide a sealant that joins surfaces contaminated with oil.

The compositions of this invention generally exhibit excellent self-adhesive adhesion even on oil-contaminated metals such as the surfaces of aluminum and steel encountered in the automotive industry. These compositions also have excellent oil resistance, such as oil resistance at elevated temperatures, eg, 300 ° F (about 149 ° C), making them particularly useful as silicone in-situ gasket materials.

The self-adhesive one-component naturally vulcanized silicone sealant formulation of this invention has a fixer of the formula:

R ¹ and R ⁶ are independently a non-aromatic hydrocarbyl or halo-substituted hydrocarbyl group of 1 to 20 carbon atoms or 1
A carboxyalkyl group of from 10 carbon atoms. R ² ,
R ³ , R ⁷ and R ⁸ are independently H, C ₁ -C ₂₀ hydrocarbyl or halohydrocarbyl, which may be aromatic or non-aromatic or C ₁ -C ₁₀ carboxyalkyl. . R ⁴ and R ⁵ are divalent hydrocarbyl or halohydrocarbyl groups of 1 to 20 carbon atoms.

Non-aromatic hydrocarbyl groups include alkyl, alkenyl,
Alkynyl, cycloalkyl and the like, preferably alkyl aliphatic or cycloaliphatic such as methyl and ethyl,
It may be a saturated or unsaturated group. Aromatic hydrocarbyl groups are aliphatic substituted aromatic groups and aromatic substituted aliphatic groups.

R ⁴ and R ⁵ are preferably C ₃ -C ₈ alkylene groups.

The numbers designated by a, b, f and e are independently 0-3, g and h are 0 or 1 and a + b + g = f +
e + h = 3.

Examples of specific fixatives are shown below.

In the above formula, x = 0 to 3 x ¹ = 0 to 3 y = 1 to 3 y ¹ = 0 to 3 x + y = x ¹ + y ¹ = 3 The actual fixing agent is often the above-mentioned formulation and / or the above-mentioned It is a statistical mixture of "general formula" formulations. Whenever formula 1 or 2 is utilized, it is also desirable to utilize compounds of formulas 3-5 so that there is a more reactive oxime group.

Various terminal segment, i.e., (R ¹ O) _a and (OR ⁶⁾ _{If e} is different from one, for example, (R ¹ O) _a has at trimethoxy (O
When R ⁶ ) _a is triethoxy, the fixative is a liquid. In other words, the asymmetric end groups yield a liquid. Therefore, it was thought that the fixative would also be a liquid if the end groups were symmetrical. However, when utilizing an end segment or group of interest such as triethoxy, the fixative is
It was surprisingly solid. This is a truly surprising result, because the end group segment constitutes a very small part of the structural arrangement. Asymmetric end segments or groups are preferred because liquids are much easier to handle.

The fixative of this invention can be made in many different ways. A preferred method of preparation is to add a compound of formula A to formula B
Is to react with the compound of.

Wherein _{^{A (R 1 -O) a -Si}} -R 4 -N = C = 0 Formula ^{_{B NH 2 -R 5 -Si- (OR}} 6) e R 1, R 4, R 5, R 6 and a, c , D and e are as described above. The reaction is spontaneous and spontaneous with the generation of heat when the two components are mixed. The reaction is generally carried out in the presence of an inert gas bracket, such as nitrogen, to avoid reacting the compound with moisture.

Generally, from about 0.95 to about 1.0 mole of Formula B is utilized for 1 mole of Formula A, although greater or lesser amounts can be utilized, but in situations where equal molar ratios are preferred. Is. The reaction is about 40 ° C to about 100
It is carried out at atmospheric pressure at a temperature of ° C. Formula B is added to A with stirring in a three-necked flask equipped with a thermometer and sealed with nitrogen.

The reaction of the compound of formula A with the compound of formula B results in the compound of formula C.

Formula C The oxime bridging unit can be grafted to the compound of formula C by heating the oxime compound at a temperature of about 45 ° C to 150 ° C, preferably about 50 ° C to about 120 ° C. Once again, an inert gas blanket, such as nitrogen, is used to prevent reaction with moisture. The type of oxime compound utilized is generally the oxime crosslinker described in Formulas Type I and Type II, or mixtures thereof.

Type I Type II R ¹⁰ is generally alkyl having 1 to 8 carbon atoms in a preferable state of methyl, alkenyl having 2 to 8 carbon atoms in a preferable state of vinyl, haloalkenyl having 1 to 8 carbon atoms, and 1 to 8 carbon atom. Is trifluoroalkyl having or haloalkenyl having 2 to 8 carbon atoms.

R ⁹ is hydrocarbyl, halohydrocarbyl or carboxyalkyl having 1 to 10 carbon atoms,
Thus, R ⁹ is preferably methyl or ethyl.

In general, the greater the number of oxime groups on the silicon atoms of the fixative, the better the adhesion. Adhesion is also affected by the R ¹⁰ group on the crosslinker. Adhesion is
It is especially facilitated when such groups are alkenyl. Often, the oxime-type crosslinker utilized to make the silicone sealant of this invention can be a mixture. About 5 to about 40%, and preferably about 10 to about 20% by weight of the oxime compound having the formulation of Type I oxime can be utilized, based on the total weight in the total formulation. Similarly, about 60 to 95% by weight of the Type II oxime, and preferably about 80 to about
90% by weight can be utilized based on the total weight of all oxime compounds in the composition. When considered on a molar basis, about 0.05 moles to 0.4 moles, and preferably about 0.1 to about 0.2 moles, of the type I oxime are added to the type I and
It is used per mol of the oxime compound of II.

The preparation of oxime compounds described in Type I and Type II formulas is well known in the art, as described in US Pat. No. 3,189,576, which is hereby incorporated by reference for its preparation. It is well incorporated.

Considering the sealant composition of the present invention, the composition contains a silanol-terminated diorganosiloxane polymer as a basic component. This linear polymer can be devolatilized by methods such as those described in U.S. Pat.No. 4,356,116, and generally has a viscosity of about 2,000.
0 to about 250,000 centipoise, and preferably,
It is about 10,000 to about 120,000 centipoise. The formula for this polymer is Where n is from about 300 to about 1,000,
R ¹ and R ² can be the same or different, but R ¹ and
R ² has an alkyl group having 1 to 8 carbon atoms when methyl is preferred, a cycloalkyl group having 4 to 7 carbon atoms such as cyclohexyl, and 2 to 8 carbon atoms when vinyl is preferred. An alkenyl group, an aryl or alkyl substituent having 6 to 13 carbon atoms such as phenyl, methylphenyl, or a fluoroalkyl group having 1 to 8 carbon atoms with 3,3,3-trifluoropropyl being preferred. is there. The amount of linear polymer generally ranges from about 25 to about 90 weight percent, but is preferably from 30 to about 60 weight percent based on the total weight of the low modulus naturally vulcanized silicone composition. Such polymers are commercially available and are available from Mobay, Union. Carbide and Watsker Chemie (Mobay, Union Carbide an
Manufactured by d Wacker Chemie).

The sealant composition comprises the fixing agent of the present invention,
It may be mixed with an oxime crosslinker and optionally with a plasticizer.

To ensure that no gelation or increase in viscosity occurs, an excess of oxime crosslinker-containing fixative to silanol in the base polymer is provided in a ratio of about 1.2 to about 4.0, preferably about 2.0 to 3.0, i.e. It is used in a molar ratio of silanol groups to the cross-linking agent.

Considering the fixative, its amount is from about 0.5 to about 3% by weight, based on the total weight of the sealant composition, preferably
It is about 0.8 to about 2% by weight.

An organotin catalyst is desirably utilized in this composition to accelerate the reaction of the oxime compound with the silicone polymer. The amount of catalyst is generally from about 0.01 to about 0.5% by weight, based on the total weight of the sealant composition, and
Preferably, it is about 0.02 to 0.2% by weight.

Examples of organotin catalysts are well known in the art and are fully incorporated herein by reference in US Pat.
6,116 and 4,395,526. Examples of specific tin compounds are dibutyltin dilaurate, dibutyltin diacetate, tin octoate, dimethyltin dibutyrate, triethyltin tartrate, tin oleate, dibutyltin oxide, and dimethyltin bis neodecanoate.

The plasticizer utilized with the linear base polymer in the sealant composition is present in an amount of from 0 to 1 to about 40% by weight, based on the total siloxane composition, and preferably 5 to about 25% by weight. A triorganosiloxy-terminated diorganopolysiloxane fluid. Plasticizer lowers the diurometer and modulus of cured rubber,
It also lowers the viscosity of the overall system or composition. But,
The plasticizer should not be too low, because too low a viscosity will tend to cause the plasticizer to bleed out of the composition. Accordingly, the viscosity is generally in the range of about 50 to 100,000 centipoise, and preferably about 500 to about 10,000 centipoise.

The plasticizer is a monofunctional triorganosiloxane end group-terminated diorganopolysiloxane polymer. The organo compound in the repeating unit is the same as R ¹ and R ² described above in this specification for the base polymer, but with a trace amount of trifunctional monoorganosiloxy building blocks derived from the additives in the starting materials. It may be contained. The siloxy building block contains an alkyl group having 1 to 8 carbon atoms with methyl being preferred. The number of repeating units in the plasticizer is generally about 20-900. Like the linear base polymer, the plasticizer is devolatilized according to conventional methods or methods well known in the art. Specific examples of devolatilization are described in US Pat. No. 4,356,116, which is fully incorporated herein by reference.

A thixotropic agent is added to the overall composition to reinforce the polymer network and provide the system with non-sagging properties. The thixotropic agent desirably adds physical strength to the system, which is preferably a treated fumed silica filler. The treated silica filler is
Generally, the water content is low, and the module property is much better and lower. The amount of silica filler is generally about 1 to about 20% by weight, with about 3 to about 8% being preferred.
Treated and untreated silica fillers are well known in the art, and generally any such conventional filler can be utilized. Examples of specific silica fillers are described in US Pat.
No. 7,878, which is fully incorporated herein by reference. Additionally, the treated silicas described in U.S. Pat.No. 2,938,009, U.S. Pat.No. 3,004,859, and U.S. Pat.No. 3,635,743 are available, all of which are incorporated herein by reference. Has been fully incorporated into. Silica fillers typically have very high surface areas, such as about 200 M ² / gram.

Optionally, about 10% by total weight of the entire strain or composition.
From 0.1 to about 5% by weight, preferably from about 0.2 to about 3% by weight of heat aging additive can be utilized. This optional ingredient is
It functions to reduce polymer oxidation and thermal rearrangement at high temperatures. These antioxidants are cerium neodecaneate,
It may also be a substance such as rare earth octoate and iron octoate. Representative examples may also be carbon black, iron oxide powder, and heat aging additives such as titanium dioxide. Of course, other pigments can be utilized as they provide different desired shades.

Another optional ingredient is a semi-reinforced or non-reinforced filler such as quartz powder, calcium carbonate, talc, clay, various silicate compounds and other materials known in the art or their treated counterparts. . Utilization is from about 5% to about 60% by weight based on the total weight of the sealant composition.

The silicone sealants of this invention are useful in that they cure at room temperature. It also eliminates the need to apply a primer precoat to the support for good adhesion. In addition, silicone sealants are low odor, glass, various supports such as ceramics, various metals such as aluminum, copper iron, etc., as well as acrylates, glass fiber reinforced polyesters and acrynonitrile butadiene styrene. Good adhesion to various plastics such as terpolymers. These sealants are also about 300 ° F (about 149 ° F).
It shows excellent oil resistance at high temperatures (° C). Therefore, such sealants are used whenever such properties are desired. Specific examples of practicality are, for example, natural vulcanizing adhesives such as gaskets, bathing caulks, joints, plastics adhesives and the like. If desired, they are used as automotive engine sealants, for example as valve cover gaskets, oil pan gaskets and the like.

Preparation of the above-mentioned silane fixing agent and kneading into the silicone sealant composition can be carried out according to one of the following two methods. The first method is the reaction product of compounds of various oxime types, and thus the exact final product is a mixture, whereas the second method produces a specific oxime product. To be done.

The first method of preparation involves the preparation of 0.5 to 4 parts of an equimolar mixture of a compound of formula A and a compound of formula B which has been previously treated. Mix up to 60 ° C for about 1 hour with the above oxime curing agent
By adding to a crosslinker mixture consisting of 12 parts and 0.2 part of the above organotin salt catalyst, this mixture is heated at 50 ° C. for 10 to 20 hours in a closed container.

The silicone sealant compound is then prepared by loading the various components into the mixture under dry conditions such as through the use of dry nitrogen to prevent hydrolysis of the oxime cross-linking compound. It depends on entering. Generally, the polymer is first added with the plasticizer. The oxime composition is mixed at high speed or with stirring and under vacuum for minutes or even hours to remove entrapped nitrogen bubbles. The resulting paste can be placed or pressed into a desired container or the like.

The present invention will be better understood with reference to the following examples.

Example 1 Wt.% 1. Silanol-terminated polydimethylsiloxane 40.06 with viscosity 10,000 centipoise 2. Trimethylsilyl-terminated polydimethylsiloxane with viscosity 1000 centipoise 15.46 3. Calcium carbonate treated with stearic acid 34.05 4. Surface area Of about 200 m ² / g of polydimethylsiloxane-terminated fumed silica 3.61 5. Iron oxide 1.40 6. Methyltris-methylethylketoximosilane 4.81 7. Dimethyltin bis neodecanoate 0.08 8. Gamma isocyanate propyltrimethoxysilane Gamma-aminopropyltriethoxysilane 0.53 This composition is used in laboratory thiendican mixers in which these ingredients were previously flushed with dry nitrogen to prevent hydrolysis of the oxime crosslinker and fixer. Charge into mixed can.

Ingredients (6), (7) and (8) were heated in a closed glass jar in an oven at 50 ° C. for 16 hours and allowed to cool before being incorporated into the sealant formulation. Subsequently, the ingredients were added as follows. That is, (1), (2),
(3), (4) and (5), and then a mixture of (6), (7) and (8). It was then mixed at high speed and under vacuum for about 2 hours, after which the resulting paste was transferred to a pressure Semco mixer. The paste was then extruded from a pressure Semco mixer into a 6 ounce polyethylene cartridge.

I found out the following. That is, 180 ° C peel adhesion of this sealant to aluminum laminated plate aluminum is 1
12 to 12 pounds per linear inch after curing for 4 days, 100% for sealant without (8)
There was 100% cohesive failure compared to 0 to 2 pounds per linear inch with adhesive failure.

In a second method of preparation, a compound of formula A and a compound of formula B are reacted at room temperature and atmospheric pressure, preferably under an inert gas blanket, and a ketoxime compound such as methylethylketoxime is obtained. Is added to the reaction product in sufficient amount and the mixture is heated to cause partial displacement of the ketoxime on the silicon atom by the elimination reaction of the atoxy component. The sealant is then prepared as in the first method of preparation.

This method of preparation can be better understood with reference to the examples below.

Example 2 44.2 grams (0.2 mole) gamma aminopropyltriethoxysilane was added dropwise to gamma isocyanatopropyl trimethoxysilane at room temperature and atmospheric pressure under a blanket of nitrogen. The addition was slow, to avoid excessive heat generation and accumulation from the reaction exotherm. When the addition was complete, 38.3 grams (0.44 mol) of methyl ethyl ketoxime was added to the reaction product and the mixture was heated to 120 ° C for about 4 hours. Analysis of the resulting gas chromatograph showed a mixture of substitution products of the methylethyl ketoximo component on the silicon atom.

Example 3 Wt.% 1. Silanol-terminated polydimethylsiloxane 40.06 with viscosity 10,000 centipoise 2. Trimethylsilyl-terminated polydimethylsiloxane with viscosity 1000 centipoise 15.46 3. Calcium carbonate treated with stearic acid 34.05 4. Surface area Fumed silica treated with about 200 m ² / g of polydimethylsiloxane 3.61 5. Iron oxide 1.40 6. Methyltris-methylethylketoximosilane 4.81 7. Dimethyltin bis neobecanoate 0.08 8. Material of Example 2 0.53 This composition Was prepared as in Example 1. The ingredients were added in the following order. That is, (1), (2),
(3), (4), (5), (6), (7), and (8), then mixed at high speed under vacuum for about 2 hours and then the resulting paste Transferred to a pressure Semco mixer and pushed into a 6 ounce polyethylene cartridge. Peel adhesion of this sealant to aluminum plywood is 100% after curing for 4 weeks.
Of 23 pounds per linear inch.

Therefore, it was found that the adhesion promoter of the present invention was used to significantly increase the adhesion.

Example 4 of Practical Use of Mixed Oxime Alkoxysilanol Urea Fixing Agent Example 4 (1) Silanol-terminated polydimethylsiloxane having a viscosity of 80,000 centipoise 39.81 (2) Trimethylsilyl-terminated polydimethylsiloxane having a viscosity of 1000 centipoise. 3) calcium carbonate treated with stearic acid 33.83 (4) Raven1020 carbon black 1.07 (5) Hiyumudoshirika 3.58 (6) vinyl tris methyl ethyl treated with polydimethylsiloxane surface area of about 200M ² / gram ketoximo silane 4.76 (7) Dimethyltin bis neodecanoate 0.08 (8) N- (3-trimethoxysilylpropyl)-
N '-(3-Triethoxylylpropyl) urea 1.49 The above composition was prepared by charging the following ingredients into a mixer under dry nitrogen gas to prevent hydrolysis.
Add (1) to the mixer, then (6), (3),
(4), (5), (2), (8) and (7) were added in this order.

The resulting paste was then made into an ASTM sheet. The test sheet was cured for 7 days. Next, the initial curing property and the oil immersion property for 2 weeks were measured. The oil immersion test was conducted after 2 weeks (14 days) at 300 ° F (about 149 ° C) in 5W-30 engine filling oil (CITGO grade). The data are summarized below.

Adhesion test coupons were also prepared using aluminum and steel panels that had been previously contaminated with a thin coating of 5W-30 engine oil. These test pieces were cured at room temperature for 2 weeks. It was found that cohesive failure (breakage in the rubber) was obtained in each case. Damage to the adhesive layer (damage from the metal support) was observed in the compound without the fixing agent. Adhesion on oil-contaminated surfaces is a clear advantage in maintaining an oil-tight seal for automobile engine flanges.

It should be noted that the fixative was not pre-balanced with the oxime crosslinker in the above examples.

Example 5 The amount of polydimethylsiloxane treated fumed silica was 3.
Example 4 was repeated except that the weight was increased from 58% to 5.09%. The following performance characteristics were measured.

Again, the same excellent adhesion results were obtained.

Claims (7)

[Claims] 1. A silanol / terminated diorganosiloxane base polymer having a viscosity of 2,000 to 250,000 centipoise at 25 ° C. and having a viscosity of 2,000 to 250,000 centipoise, wherein the organo groups may be the same or different. 1-8
Alkyl having carbon atoms, cycloalkyl having 4 to 7 carbon atoms, alkenyl having 2 to 8 carbon atoms, 6
25-90% by weight, based on the total weight of the composition, of a polymer selected from the group consisting of aryl- or alkyl-substituted aryls having 14 carbon atoms and fluoroalkyls having 1-8 carbon atoms; (b) triorganosiloxy. A volatile-free diorganopolysiloxane liquid deprivation agent having a terminal group, wherein the organo group has alkyl having 1 to 8 carbon atoms, cycloalkyl having 4 to 7 carbon atoms, and 2 to 8 carbon atoms. Selected from the group consisting of alkenyl having, allyl or alkyl substituted allyl having 6 to 14 carbon atoms and fluoroalkyl having 1 to 8 carbon atoms at 25 ° C.
A plasticizer having a viscosity of 50-100,000 centipoise at 0-40% by weight, based on the total weight of the composition; and (c) Formula: Where R ¹ and R ⁶ are independently a non-aromatic hydrocarbyl or halohydrocarbyl group of 1 to 20 carbon atoms; R ² , R ³ , R ⁷ and R ⁸ are independently H, aromatic Or a non-aromatic C ₁ -C ₂₀ hydrocarbyl or halohydrocarbyl; R ⁴ and R ⁵ are divalent hydrocarbyl or halohydrocarbyl groups of 1-20 carbon atoms; a, b, e
And f are independently 0 to 3, g and h are O or 1
And a + b + g = f + e + h = 3. ) 0.5 to 3% by weight of the adhesion promoter having: And mixtures thereof (wherein R ⁹ is hydrocarbyl,
Halohydrocarbyl or carboxylalkyl having 1 to 10 carbon atoms; and R ¹⁰ is alkyl having 1 to 8 carbon atoms, alkenyl having 2 to 8 carbon atoms, 1
Haloalkyl having 8 carbon atoms, trifluoroalkyl having 1 to 8 carbon atoms or haloalkenyl having 2 to 8 carbon atoms. Room temperature vulcanizable silicone rubber composition comprising an excess mole of silanol of the crosslinking agent selected from the group consisting of: 2. The room temperature vulcanizable silicone rubber composition according to claim 1, wherein g and h are zero. 3. The room temperature vulcanizable silicone rubber composition according to claim 2, wherein one of R ^{1 and} R ⁶ is methyl and the other is ethyl. 4. A room temperature vulcanizable silicone rubber composition according to claim 1, further comprising an effective amount of a thixotropic agent that imparts non-sagging properties to the composition. 5. The room temperature vulcanizable silicone rubber composition according to claim 1, further comprising an organotin catalyst. 6. The catalyst is selected from the group consisting of dibutyltin dilaurate, dibutyltin diacetate, tin octoate, dimethyltin dibutyrate, triethyltin tartarate, tin oleate, dibutyltin oxide and dimethyltin bis neodecanoate. The room temperature vulcanizable silicone rubber composition according to claim 5. 7. The room temperature vulcanizable silicone rubber composition according to claim 1, further comprising 1 to 60% by weight of a filler.